1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same and is applicable to photographing systems such as digital cameras, video cameras, and silver-halide film cameras.
2. Description of the Related Art
A so-called negative lead type zoom lens has been known, in which a lens unit of negative refractive power is located on the most object side. Negative lead type zoom lenses are commonly used as wide-field-angle photographing lenses because the close-up photographing distance is comparatively short, a wide field angle can be comparatively easily achieved, and the back focus can be easily increased.
A zoom lens including, in order from the object side, negative, positive, negative, and positive lens units is known as a negative lead type zoom lens for single-lens reflex cameras (U.S. Pat. No. 5,710,669 and Japanese Patent Laid-Open No. 2006-58584).
In U.S. Pat. No. 5,710,669, the first lens unit is separated into a front unit of negative refractive power and a rear unit of negative refractive power, and the rear unit is moved during focusing. U.S. Pat. No. 5,710,669 discloses a zoom lens having a super wide field angle of about 100° at the wide-angle end.
The zoom lens of Japanese Patent Laid-Open No. 2006-58584 has the same lens configuration as that of U.S. Pat. No. 5,710,669. The front unit of the first lens unit consists of two negative lenses. This improves optical performance and achieves a super wide field angle of 110° or more.
In recent years, zoom lenses for digital single-lens reflex cameras have been strongly required to be compact, to have a wide field angle, and to produce a high-quality image.
In general, a negative lead type zoom lens is advantageous to achieving a wide field angle but has a significantly asymmetrical lens configuration. Therefore, when each lens unit moves during zooming, aberrations fluctuate significantly due to asymmetrical change in lens configuration. Therefore, it is difficult to achieve high optical performance throughout the zoom region. In particular, when the lens configuration of the most object-side negative lens unit is not appropriate, it is difficult to achieve high optical performance throughout the zoom region while achieving a wide field angle. For example, off-axis aberrations, especially distortion and astigmatism, are significantly generated at the wide-angle end, and it is difficult to effectively correct these aberrations. In order to effectively correct these aberrations, a large number of lenses are necessary, and therefore the size of the whole lens system is increased.